This invention relates to an interlock vessel for use in a hyperbaric transfer system. Deep sea diving, whether for pleasure or work, is associated with a serious risk of trauma to the divers. Without proper treatment, major problems from diving accidents, most commonly Decompression Sickness (the “Bends”) and Air Embolism, can lead to permanent disabling injuries and in some instances be fatal. Conventionally, offshore rig divers who work at great depths for considerable amounts of time must undergo decompression for periods up to two weeks. Normally, the decomposition takes place in a conventional decompression chamber on the offshore rig or on a deck of a dive boat. However, in rig abandonment situations or in situations in which a diver is seriously injured, it may be necessary or desirable to leave the offshore decompression chamber. In such cases, the divers undergoing decompression must be transferred from the offshore rig to another hyperbaric facility.
Dive chambers are examples of a category of pressure vessel referred to as a PVHO (i.e.—pressure vessel for human occupancy). Once the divers are inside the vessels of the transfer system, their condition must be kept stable. In keeping with this objective, the problem arises of keeping the gas mixtures constant within the vessels of the transfer system. This includes both the pressures and concentrations of the compression gas, the breathing gas and the oxygen within the chamber. It is especially true for the oxygen supply within the vessel which must be replenished as it is used.
Conventionally, the oxygen could be regulated by feeding oxygen into the vessel and providing the vessel with an oxygen analyzer which would measure the gas concentration within the vessel. Similar analyzers and meters could be provided for compression or breathing gas mixtures. However, the process of feeding gas into the chamber, waiting for the pressure or concentration within the vessel to stabilize and reading the analyzer is slow and requires the complete attention of the individual performing the operation. In an emergency situation, such as a fire on the offshore rig, the time necessary to take an accurate measurement is not available. The persons moving the vessel have all they can do just moving the vessel or removing the vessel from the offshore rig. Furthermore, in an emergency situation, there is no assurance that personnel capable of accurately metering gas into the vessel and reading the analyzer will be available.
While the diver is in the decompression chamber, if medicines or supplies must be passed to the diver, an air lock must be used. The air lock on a dive chamber consists of a steel tube which penetrates the wall of the dive chamber. The steel tube has a door called a “closure” on each end. An air lock on a pressure vessel for human occupancy (e.g. a decompression chamber) should be able to be operated quickly and easily, should be able to accommodate moderate wear without catastrophic failure and should have an interlock so that the operator's actions are reasonably constrained. If the closure is economical that is an advantageous feature also.
The transfer chamber has specific requirements. For instance, the exterior closure must withstand the internal pressure of the dive chamber when the inner door is open. A device called a quick opening closure is suitable for this purpose. An economical choice for this small diameter application is a breech-lock type “two-ring” design familiar to those skilled in the art of quick opening closures. A two-ring style door uses a body ring welded to the body of the air lock and a “moving ring” which is the door. The door ring has radial lugs pointing outward. The body ring has radial lugs pointing inward. When the door ring is swung into the body ring and rotated, the door lugs engage their companion lugs on the body ring. Because the mating surfaces of these lugs are sloped, the relative rotational motion of the lugs on the two rings causes the door ring to be drawn toward the body ring, and thereby energize the seal which is between the two rings.
Such two-ring closures are in contrast to “three-ring” closures in which the door and body ring are non-rotating, but a third ring outside of those two rings (a lock ring) rotates to engage mating lugs on the door and/or body ring and thereby affect a seal. Compared to three-ring closures, two-ring closures are beneficial as they do not have the expense of the third ring, do not require lubrication of the sliding surfaces of the third ring, and do not have their high stress areas hidden under a third ring. These advantages are particularly useful in a competitive commercial application such as a dive chamber where the closure is subjected to accelerated aging caused by an outdoor marine environment.
The two-ring doors are not without their problems. One of the hazards associated with any manually operated quick opening closure is that the operator can attempt to operate the closure while it is under pressure. As a consequence, the operator can be injured or an injury can be inflicted on a diver positioned in the small-size decompression chamber. The general methods used to prevent the two-ring doors from being opened while under pressure rely on indicators or interlocks. Examples of “indicators” are a pressure gage or pressure actuated spring loaded pop-up piston. Indicators only notify the operator and depend on his recognizing and acting on the information which the indicator is presenting. Also, spring-loaded piston indicators retract when a small pressure still remains in the chamber so that a false “OK” signal can be communicated.
Another disadvantage of two-ring doors is related to the door support because the door not only swings out, but also rotates about its axis. Because of this, the a two-ring door hinge typically connects to the door via a bearing in the hinge blade which supports an axle in the center of the door. Bearings eventually wear and that allows alignments to change. This alignment is relevant because O-rings are the preferred sealing devices for quick opening closures because they are economical and readily available, and because they are in a class of gaskets referred to as “self-energizing.”
Self-energizing gaskets use the pressure of the fluid they are retaining to contribute to their sealing force. O-rings seals do require containment in a cavity with limited gaps to prevent a form of failure referred to as “extrusion.” Extrusion failure of O-rings and the design gap sizes required to prevent it are described in O-ring design handbooks such as the “Parker O-Ring Handbook” and are familiar to those skilled in the art of O-ring joint design. For a closure where human life depends on its proper operation, a concentricity misalignment of the door which leads to a gap and possible extrusion failure is unacceptable.
An interlock is a device which constrains the operator from opening the door until after a vent has been opened. An example of a previous solution for a two-ring door would be a threaded vent plug in the door which is chained to a stationary part of the vessel. Compared to three-ring closures, small two-ring closures are more of a design challenge to interlock because the motion of the door needs to be constrained relative to the venting of the chamber. “Vent plug-on-chain” interlocks constrain behavior, but they are slow and awkward.
Another problem with dive chamber air locks relates to the operation of the inner closure or door. The inner door swings inwards. As a result a pressure differential between the living space and the air lock chamber presses the inner door against the seal between it and the air lock body tube. The air lock inner door, therefore, does not need a closing mechanism when a pressure difference exists. However, dive chambers are utilized on ships which can have large motions, and they are frequently moved to the next dive job location on trucks. During these periods an unsecured door will bounce open and closed and possibly cause damage. Also, while on the deck of a vessel that is listing (for example while discharging a portion of its cargo) an unlatched inner door can swing open on its own.
If the inner door does open by itself when the dive chamber is pressurized but unoccupied, the operator standing outside cannot reach through the outer door to close the inner door because the pressure on the air lock outer door cannot be isolated from the pressure in the dive chamber. The inconvenient remedy is to release the pressure in the dive chamber so that the problem can be addressed. A seemingly simple solution is a swing bolt latch or other clamping latch on this inner door. But, this has the disadvantage that it can hold the door closed and trap pressure inside the air lock as the living space of the dive chamber is reduced during the depressurization treatment. Such a condition could lead to an explosive release of the inner door upon the failure of this latch.